US 8218212 B2
A device for the production of holographic reconstructions having light modulators is disclosed. The device comprises at least one pixelated light modulator illuminated by at least one light source, and a focusing optical element field arrangement which images the light sources in an image plane after the light modulator. For the reconstruction, only one order of diffraction of the Fourier spectrum of the hologram should be used. The light modulator is provided with an assigned filter-aperture field arrangement which is located in the area of the image plane of the light source images and which has a plurality of aperture openings. Said aperture openings are designed in such a way that they each allow the passage of a prespecified area of the overall dimensions either smaller or the same as a diffraction order of the diffraction spectrum following Fourier transformation and produced from the holographic coding of the light modulator.
1. Device for generating holographic reconstructions with at least one light modulator, comprising:
A light source or a light source array comprising multiple light sources,
At least one pixelated light modulator, which is illuminated by at least one light source,
A first focussing optical element array, where each optical element is assigned to a group of encodable pixels of the light modulator, and where the optical elements image the at least one light source into an image plane behind the light modulator so to form light source images,
A second focussing optical element array having optic elements being assigned to the optic elements of the first focussing optical element array and being arranged in a distance of their focal length behind the image plane such that they project the light bundles emanating from them to an eye of a viewer into a viewing region, and
A control unit, which is connected to the light modulator, and which computes with the help of programming means the holographic code for the pixelated encoding surface of the light modulator,
wherein the light modulator is assigned with a filtering aperture array which has a multitude of apertures, and which is situated near the image plane of the light source images and which forms together with the first focussing optical element array and the second focussing optical element array a 4f-filter-system, and whose apertures in the filtering aperture array are formed such that they let pass a defined region of the diffraction spectrum which has been generated by holographic encoding of the light modulator, said defined region having a size that is smaller than or identical to one diffraction order of the Fourier transform.
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This application claims the priority of PCT/EP2008/054584, filed on Apr. 16, 2008, which claims priority to German Application No. 10 2007 019277.2, filed Apr. 18, 2007, the entire contents of which are hereby incorporated in total by reference.
The present invention relates to a device for generating holographic reconstructions with light modulators, comprising:
The term ‘pixelated light modulator’ shall not necessarily be understood in the context of this invention as a modulator which comprises an arrangement of discretely controllable elements. It can also be a modulator with a continuous encoding surface, which is formally divided into discrete elements by the information to be displayed.
Further, the term ‘optical elements’ shall not necessarily be understood to be or to comprise conventional glass lenses, but it can be construed in a wider sense to be or to comprise refractive or diffractive optical elements which fulfill the same function.
A device for generating holographic reconstructions of representations, in particular three-dimensional scenes, is described in document WO 2006/119920 A1.
If information for example of a computer-generated hologram is stored on the pixelated light modulator, and if the light modulator is illuminated with sufficiently coherent light, a reconstruction of a three-dimensional scene will be generated in a reconstruction space. However, undesired periodic continuations also occur in the form of higher diffraction orders, because of the discrete representation of the hologram in the light modulator. Depending on the hologram encoding method employed, undesired regions can also occur within a diffraction order, which must therefore be filtered.
A conventional method for eliminating disturbing diffraction orders is to use a filter unit, e.g. a 4f arrangement, which can filter such diffraction orders. The filter unit can be dimensioned such that it only lets pass regions which are smaller than or identical to one diffraction order.
Such a method is applied for example in document DE 10 2005 023 743 A1. This document describes a holographic projection device and a method for generating holographic reconstructions of scenes using one-dimensionally and two-dimensionally encodable light modulators, said device comprising a light source, a focussing optical system, the corresponding light modulator, a projection system, and a filtering aperture, which is arranged between the light modulator and the projection system, and which lies in the image plane of the light source image.
The focussing optical system represents for the light modulator an optical illumination system, and for the light source an optical imaging system which images the light source into the image plane of the optical illumination system, where the Fourier transform of the light modulator is simultaneously generated in the image plane of the light source.
The projection device comprises a control unit which does not only encode the light modulator dynamically, but which also tracks the visibility region and thus the holographic reconstruction to a changing observer position. To achieve this, a position detection system is provided, which is connected to the control unit. The code on the light modulator is modified such that the reconstruction of the three-dimensional scene appears in a horizontal, vertical and/or axial position horizontally and/or vertically displaced and/or turned by an angle, according to the actual observer position.
In a dimensioned modification of the size relations of the above-described projection device in the form of a large, observer-friendly direct-view device, e.g. with a display with a diagonal of 20 inches, which is the size of a typical desktop monitor, a filtering process is performed on the light modulator, where a single light source is provided for the coherent illumination of the entire light modulator in conjunction with a filter unit. The direct-view device with the 20-inches display can comprise the light source, a focussing optical system, the corresponding light modulator, a projection system, and a filtering aperture, which is arranged between the light modulator and the projection system, and which lies in the image plane of the light source image. The filtering aperture comprises an opening which only lets pass the desired one diffraction order of the Fourier transform of the light modulator. The projection system images the aperture into another plane, which represents the observer plane at the same time. The observer in the observer plane can see the holographic reconstruction in a visibility region which corresponds to one diffraction order of the Fourier spectrum.
The corresponding filter unit requires in addition to the filtering aperture at least two lenses of which at least one is about as large as the light modulator that represents the display. This means for example in the case of the holographically encoded 20-inches display panel that one lens must have a diameter of at least 40 centimeters.
Because lenses typically only exhibit an adequate image quality at a ratio of focal length and aperture of much larger than one, and because the filtering takes place at the position of the light source image, here in the focal plane of the first lens, a filter unit first wide lens, filtering aperture, second wide lens—which has a depth of substantially larger than 40 centimeters in front of the light modulator panel is required in this example. In the direct-view device with a light modulator panel as a screen, if a large display is used (e.g. with a diagonal of 20 inches), it is rather complicated to provide a wide lens which has about the size of the screen, where, in addition, the filter unit has a very large depth, as described.
One problem is that the design of a holographic direct-view device with the described dimensions of the optical components is very voluminous and heavy, which is undesired.
A further problem is that, in display holography, because of the pixel dimensions of commercially available light modulators, only very small useable diffraction angles are provided, which, in turn, cause a small observer window.
According to a method of display holography described in document U.S. Pat. No. 3,633,989, HPO (horizontal-parallax-only) holograms are used, where a hologram encoding is only performed in one dimension. Values for the one-dimensional hologram are computed independently of each other and are typically written to individual rows of a light modulator. In order to increase the diffraction angle, hologram values, which are typically encoded in multiple pixels arranged side by side, can in this case be encoded in pixels which are arranged below one another in multiple rows.
When using one-dimensional holographic codes within the light modulator, it will only be possible for a one-dimensional holographic reconstruction to take place. The light wave diffracted by the one-dimensional HPO hologram of the light modulator accordingly extends in the horizontal direction in the visibility region.
It is therefore the object of the present invention to provide a device for generating holographic reconstructions with light modulators, said device being designed such that an expensive arrangement at least of the optical system can be avoided on the one hand and that the diffraction angle which is used for the visibility region can be increased, on the other hand. The dimensions of the device in the axial direction shall be kept as small as possible.
The object is solved with the help of the features of claim 1.
The device for generating holographic reconstructions with light modulators comprises:
A light source with an optical beam widening system can be arranged in front of the light modulator for illuminating the light modulator.
A dynamic shutter modulator can be provided between the optical beam widening system and the focussing optical element array.
As an alternative for illuminating the light modulator, a light source array with a multitude of light sources can be disposed in front of the light modulator.
The device can comprise a light source array, a first optical element array as an optical beam widening system, and a second optical element array with multiple spherical optical elements, e.g. in the form of spherical lenses, as a screen for the observer.
A power supply unit is assigned to the light source or the first light source array.
The control unit for encoding the light modulator is a part of a control system, which also comprises a unit for controlling the light source array, and/or a unit for controlling the filtering aperture array, and a position detection unit for detecting the actual observer position.
The position detection unit can be connected to the two units; at least by signal.
The two units can optionally be connected to a displacing device which displaces in their respective planes the light sources of the light source array, and/or the filtering apertures of the filtering aperture array, which represent the movable components, in response to signals from the position detection unit. However, the first and the second optical element array can also be of a displaceable design.
Both the light source array and the filtering aperture array can be designed either as static components, or as dynamic optical components which are adjusted by the control system.
The pixelated encoding surface of the light modulator can for example have pixels of a square design.
The first optical element array represents for the light modulator an optical illumination system, and for the light source array an optical imaging system which images the light source array into the focal plane which is given as the Fourier plane of the light modulator, where the images of the light source array coincide with the Fourier transform of the pixels of the respective subsection of the light modulator through which the light shines, and where the filtering aperture array which lets pass the given diffraction order is disposed near the focal plane.
The filtering aperture array can exhibit a grid of apertures which only let pass the given diffraction order of the Fourier transform, or only parts thereof.
The projecting second optical element array with the particularly two-dimensional spherical lenses images the apertures of the filtering aperture array into a second plane, which serves as the observer plane at the same time. Optical elements and filtering apertures are mutually arranged such that the images of all apertures overlap in the observer plane, thus forming an observer window.
The first optical element array can be a two-dimensional arrangement of spherical lenses which are disposed downstream the point light sources of the light source array.
A single spherical lens of the first optical element array and a single spherical lens of the second optical element array can have a size which typically ranges between about three and ten millimeters.
The size of the apertures of the filtering aperture array depends on the pixel pitch p of the light modulator and on the focal length of the lenses of the first optical element array.
The filtering aperture array can be a shutter modulator whose controllable openings have the dimensions of one or multiple pixels of the shutter modulator.
The programming means for encoding the pixels of the light modulator in the control unit can be adapted to the design of the device according to the present invention.
If HPO holograms are used, the hologram values can be encoded in multiple horizontally or vertically adjacently arranged pixels of one or multiple rows of the light modulator.
In the control system, in particular in the control unit, it is possible to carry out a holographic encoding in only one dimension, where the values written to a group of rows or columns of the light modulator are related to each other.
The first optical element array can then be a lenticular array with cylindrical lenses, which is illuminated by line light sources and which is assigned with a filtering aperture array with slotted apertures.
A sufficiently coherent illumination of the light modulator must then only be achieved in the area of the group of a few rows.
In order to track the visibility region to the observer, a dynamic shutter modulator for displacing the position of the apertures can be used as a filtering aperture array.
The light source array can comprise an arrangement of adjacent light sources which can be turned on individually one after another, where said arrangement illuminates a certain vertical section in a certain interval of time, which can be controlled by the control system.
In order to enlarge the visibility region used by the observer, particularly in the vertical direction, diverging lenses can be used, where the entirety of diverging lenses can also have the form of a diverging lens array, which is disposed directly downstream the filtering aperture array.
Optionally, depending on the design and encoding method used for the light modulator, one-dimensional, slotted filtering aperture arrays or two-dimensional filtering aperture arrays with round apertures can be employed.
The filtering aperture array can be designed statically in the form of an aperture mask.
In order to track the visibility region or to periodically scan a certain viewing range, a dynamic filtering aperture array can be provided which is realised with the help of the controllable displacing devices of the control system.
The filtering aperture array can be a fast switching amplitude-modulating light modulator where the variation of the transmittance of individual pixels causes a filtering effect, where the activated pixels, which then serve as apertures, roughly correspond to the size of the opening of the apertures of the static filtering aperture array.
The light source array can, in agreement with the dynamic filtering aperture array, be a fast switching amplitude-modulating light modulator, which is entirely illuminated by a light source, and where the variation of the transmittance of individual pixels causes light beams to be let pass, where the pixels, which then serve as openings for beam passage, have about the size of the diameter of the light sources of the static light source array.
The present invention will be described in more detail below with the help of a number of embodiments and drawings, wherein:
According to the present invention, the light modulator 2 is assigned with a filtering aperture array 8 which has a multitude of apertures 81, and which is situated near the image plane 6 of the light source images 42, and whose apertures 81 in the filtering aperture array 8 are formed such that they let pass one specific diffraction order or parts thereof of the diffraction spectrum which has been generated by holographic encoding of the light modulator.
The inventive device 1 according to
The first optical element array 5 can be a two-dimensional arrangement of spherical lenses 51 which are disposed downstream the point light sources 41 of the light source array 4, where a two-dimensional filtering aperture array 8 of apertures 81 and a second optical element array 13 are provided as well.
A single lens 51 of the first optical element array 5 and a single lens 131 of the second optical element array 13 can for example have a size which typically ranges between three and ten millimeters.
The total depth of the device 1 in the z direction only increases moderately due to the filtering with the arrays 4, 5, 6, 13, and is much smaller than the dimensions of the arrangement involving wide lenses which are described in the prior art section.
The filtering aperture array 8 here is a two-dimensional grid with small openings, namely the apertures 81. The size of the apertures 81 depends on the pixel pitch p of the light modulator 2, as shown in
The filtering aperture array 8 can alternatively be a shutter modulator with controllable openings which have the dimensions of one or multiple pixels of the shutter modulator.
The programming means for the holographic encoding of the pixels 21 of the light modulator 2 in the control unit 7 can be adapted to the design of the device 1.
In order to reduce the required hologram computing time, HPO (horizontal parallax only) holograms are used in prior art display holography, where the hologram is only encoded in one dimension, e.g. in the y direction, as shown in
Here, the first optical element array 5 and/or the second optical element array 13, as shown in
In order to enlarge the diffraction angle and thus the useable visibility region in the plane 61, it can be possible in the case of an HPO hologram for example to use a combination of multiple rows of a hologram, instead of multiple columns, in order to encode a complex hologram value.
One possibility for the computation in the control unit 7 is here for example a representation of a complex number by multiple phase values, where the one-dimensional arrangement of complex hologram values is computed in the horizontal direction, i.e. in the y direction, while the phase values which form a complex number are arranged in pixels one above another in the vertical direction. To achieve this, a coherent illumination is only required for a group 28 of a few rows 24, 25, 26, 27. If a group 28 of rows 24, 25, 26, 27 of a light modulator 23 is coherently illuminated, this will cause in the vertical direction, i.e. in the x direction an undesired retardation of optical path among the individual rows, where said retardation is angle-specific, and leads to a deviation of the expected reconstruction.
While filtering units of a 4f-arrangement type according to
The complex amplitude and phase values on the light modulator 23, as shown in
In the vertical direction, the desired signal itself, as a coherent addition of multiple light modulator rows 24, 25, 26, 27, is transmitted (or undesired portions thereof are filtered out) in the image plane 6, and not its Fourier transform. However, an observer 14 must also be able to move vertically within the visibility region in the plane 61, so that he can watch the original reconstruction 91, or the accordingly displaced reconstruction 92, from multiple vertical positions, as shown in
However, a preferred alternative for adjusting the visibility region in the plane 61 to the observer 14 can be a dynamic shutter for displacing the position of the apertures 81 or 82 in the filtering aperture array 8. This can be achieved in conjunction either with a modification of the values represented on the light modulator 2, 23—for example by adding a certain phase offset for an entire row when employing a phase encoding method—or with a movable light source array 4. This has the advantage that a light modulator 2 with comparatively slow switching speed can be used as well.
Referring for example to
In conjunction with a light source array 4, the device 1 according to the present invention allows undesired diffraction orders to be filtered out for each single section of a hologram, which is illuminated with sufficient coherence by a light source 41. This particularly allows small, compact filter units to be used, which can also be disposed in front of a large holographic screen 13. Optionally, depending on the design and encoding method used for the light modulator 2, 23, one-dimensional directed —preferably slotted —filtering aperture arrays 8, or two-dimensional filtering aperture arrays 8 —preferably with round apertures —can be used.
The filtering aperture array 8 can be static, in the form of an aperture mask.
A further embodiment of the device 1, which allows a certain visibility region in the plane 61 for the observer 14 to be tracked or to be scanned periodically, is the dynamic design of the filtering aperture array 8 through the controllable displacing devices 20 of the control system 16.
The filtering aperture array 8 can then for example be a fast switching amplitude-modulating light modulator where the variation of the transmittance of individual pixels or pixel groups effects a filtering. The pixels or pixel groups, which can then serve as apertures 81, then have about the size of the opening of the apertures 81. Because the individual filter units of the filtering aperture array 8 are illuminated by light sources which are incoherent in relation to each other, no new diffraction structure will be created by the filtering aperture array 8.
The light source array 4 can, in agreement with the filtering aperture array 8, be a fast switching amplitude-modulating light modulator, where the variation of the transmittance of individual pixels or pixel groups causes light to be let pass, where the pixels or pixel groups, which then serve as openings for light passage, have about the size of the diameter of the light sources 41 of the static light source array.
A preferred application of the filtering aperture array described above is to filter out an angle-dependent phase shift among pixels, which is not desired but cannot be avoided when encoding complex hologram values in multiple adjacent phase pixels. This undesired phase shift, which occurs in addition to a programmed, desired phase shift, is caused by the fact that the pixels which represent one hologram value are arranged side by side and not one behind another. This will now be explained with the example of an embodiment where the optical element arrays 5 and 13 and the filtering aperture array 8 are understood to form a 4f filtering arrangement, and where one complex hologram value is encoded with the help of mere phase values in two adjacent pixels.
The first optical element array 5 comprises focussing lenses as optical elements 51, and the second optical element array 13 also comprises focussing lenses as optical elements 131, where the two optical element arrays can be designed in the form of lenticular arrays.
Two pixels 291, 292 each form a group or macro pixel 29 for the two-phase encoding of the complex hologram value, where the macro pixel 29 has the same size as the lenses 51. The size of the lenses 51 is exemplarily given as 60 μm in
In addition to the shown phase values of the two single pixels, a further, illumination-angle-dependent phase shift would occur between the two pixels if they were illuminated at an oblique angle, because the pixels are disposed side by side. This additional phase shift would falsify the desired complex value, but it is filtered out by the 4f filtering for each pixel group, so that the macro pixel 32 indeed exhibits the desired phase and amplitude value at the exit of the 4f system.